Sound recording system



May 10, 1938. H1 R N ET AL 2,117,214

SOUND RECORDING SYSTEM Filed July 16, 1956 2 Sheets-Sheet l y 10, 1938. HMOUNDET'AL ,1 7

SOUND RECORDING SYSTEM Filed July 16, 1956 2 Sheets-Sheet 2 Patented May 10, 1938 UNITED STATES PATENT OFFICE SOUND nnoonnmc SYSTEM Application July 16, 1936, Serial No. 91,006 In Great Britain July 19, 1935 12 Claims.

This invention relates to sound recording systems and more particularly to a system of recording sound on photographic film.

In recording sound on film various expedients have been proposed for reducing the background noise which occurs during the reproduction of sound passages of. relatively low volume and level including that of employing a number of sound tracks in parallel, recording taking place on one or more of these tracks in dependence upon the sound intensity to be recorded. Proposals of this kind involve, however, the necessity of changing the amplitude of the sound signals recorded upon each track at the instant of change over from a single track to two tracks or from two tracks to three, the necessary alteration in amplitude being effected by varying the gain of the amplifier feeding the recording oscillograph or other recording device.

The present invention has for its object to provided an improved multi-track sound recording system wherein the necessity of changing the gain of the sound signal amplifier as the number of tracks utilized varies is avoided.

25 The invention may be carried out in a number of different ways but the underlying principle in every case will, it is believed, be understood from the following explanation.

If sound be recorded on two exactly similar tracks and if. reproduction be effected from one only of these tracks the reproduced sound intensitywill have a certain value A. If nowthe second track be switched in the .sound intensity will be increased to 2A. It will beseen, however, that 35 if a third track be employed upon which the same sounds are recorded in the opposite sense to those on the second track and with equal amplitude the second and third tracks may be switched in or out without changing the total reproduced 40 volume. Now in the ordinary variable area method of sound recording a beam of light which is focussed to a slit or line upon the film is oscillated in accordance with the sound waves to be recorded at right angles to the direction 45 of traversal of the film past the recording point but only one end of said slit is used to produce the cutting edge of the sound track the opposite edge being bounded by a straight line produced by an obturating knife edge which cuts 011 50 a portion of the light beam. Thus a sound film recorded by the ordinary variable area method appears as represented (purely diagrammatically) in the accompanying Fig. 1 which represents a length of sound film record (such as might 55 be along one edge of a moving picture film) of the ordinary variable type, the shaded portion 1 being the exposed portion on the record (black in a positive film) and the wavy line 2 being the track of the cutting edge. If, however, the whole of the oscillated slit or line of light were utilized for recording, a double sound track would be produced said track being of constant width so that no sound would be reproduced therefrom. The accompanying Fig. 2 represents such a sound record, there being two cut- 10 ting edge tracks, parallel to one another with one exposed portion la between them. In other words there have been produced two mutually cancelling sound tracks. If, however there were obtained what is in effect a combination of the 15 records of Figures 1 and 2-i. e. if there were three, or any other odd number of parallel cutting edge trackssound corresponding to that originally recorded could be reproduced just as in the case of the record represented in Fig. 1 0 and the track or exposed portion between the second and third cutting edge tracks-or between the fourth and fifth; sixth and seventh and so on, if there be more than three cutting edge tracks-could be faded or shaped in and out so required for the purpose of minimizing ground noise without changing the volume of reproduced sound. The accompanying Fig. 3 represents diagrammatically a length of sound film record which is in efiect a combination of a record as shown in Fig. 1 with a record of the general nature of that of Fig. 2. In Fig. 3 there are five cutting edge tracks 2, 2a 2b, 2c, and 201, (all parallel) and three exposed portions or tracks I, la and lb. The main track I (which is never switched out) is bounded on one side by the line 2. The lines 2a and 2b define track la and the lines 20 and 2d define track lb. It will be appreciated that tracks Ia and [h can be faded in and out (as represented at points X) or shaped in and out as may be required in dependence upon sound level (being brought in one after the other for weak sounds and taken out for loud sounds) without altering the volume of reproduced sound. Fig. 4 shows a record similar to that of Fig. 3 except that the additional tracks are shaped in and out instead of being faded in and out. Figs. 3 and 4 illustrate the principle of the invention and show also two records made in accordance therewith.

According to this invention a variable area method of eiTecting so-called noiseless sound recording is characterized in that cutting edges" are brought into and out of use in pairs so that the efiect (in sound reproduction from the recorded film) of any one of said cutting edges in the pairs is balanced out by the effect of the other cutting edge of the pair in question, there being always either one or some other odd number of cutting edges in use.

In one way of carrying out the invention the number of sound tracks or exposed portions is varied in dependence upon the sound level, the additional sound track or tracks which is or are brought into use for weaker sounds, being each between a pair of parallel cutting edge tracks corresponding with the sound, the remaining track (which is not switched in and out) bein bounded only on that side adjacent said additional tracks by a further cutting edge track which is parallel to all the other cutting edge tracks, the sound tracks being spaced short distances from one another and side by side. Though the invention is not limited to the use of any particular number of tracks, there may conveniently be two additional or double sound tracks (this is the case illustrated in Figures 3 and 4) in addition to the main sound track (track I of Figures 3 and 4) and when the amplitude of the sound being recorded exceeds a predetermined value, the first (track lb of Figs. 3 and 4) of these double sound tracks is faded-out (Fig. 3) or shaped out (Fig. 4) and for very loud passages the second double sound track (track la) also is faded-out or shaded out, thus making available the whole recordable width on the film for the large amplitude oscillations of the light slit which produces the main sound track (track I).

In one form of apparatus suitable for use in recording sound by the method of this invention the image of a vertical edge illuminated by a powerful lamp is concentrated in the horizontal direction into a thin line of light and projected and focussed on to the film by reflection from the mirror of an oscillograph energized in accordance with the sound to be recorded, the line of light on the film being transverse to the direction of motion of the film and having that end corresponding to the said vertical edge positioned (in the absence of acoustic input to the oscillograph) centrally of the available record space on the said film. In addition to this Vertical edge there are four more edges all parallel to the said vertical edge. The arrangement of these edges is represented in the accompanying Fig. 5a where E represents the first mentioned edge and Ea, Eb, E and Ed the four other edges. The edge E corresponds to the normal edge (such as would produce the cutting edge track 2 of Fig. 1) of ordinary variable area recording, the diaphragm, mask or the like having this edge being long enough to cut off all light from reaching the oscillograph mirror (and hence the film) except that passing the edge Obviously if the light beam projected on to the oscillograph is in the position represented by the chain line rectangle P in Fig. a. an ordinary variable area sound record will result; if the light beam be moved into the position represented at Q there will be added an additional track bounded by two parallel cutting edge tracks; and if the light beam be moved to R there will be added a further additional track bounded by two more cutting edge tracks. This last condition is represented in Fig. 5b, shown vertically above Fig. 5a, the cutting edge tracks being marked 2 (corresponding to edge E) 2a and 212 (corresponding to edges Ea and Eb) and 2c and 2d (corresponding to edges E c and Ed). Fig. 5b represents, of course, the negative film. Fig. 5c, vertically above Fig. 5b, represents the corresponding positive film. For the sake of simplicity the cutting edge tracks in Figs. 5a and 5b are shown as straight lines. In practice, of course, they would be sound wave lines (as shown in Figs. 3 and 4) due to the acoustic frequency movements of the oscillograph mirror.

The necessary relative movement between the light beam and the means providing the edges E--Ed is obtained in any convenient way in dependence upon sound level. For example the edges EEd may be provided by a mask in the form of a thin strip of mica or film with the edges photographically produced thereon, the mask therefore, appearing as shown in Fig. 5a. Preferably the end edges M and N are shaded off so that the sound tracks are gradually faded out as the mask is moved to and fro to bring the light beam into and out of such positions as shown at P, Q and R in Fig. 5a. Such a strip of mica or film mask may conveniently be about one inch long and about inch wide. To the centre of the mica or film strip is fixed a moving coil suspended in a magnetic field and having a natural periodicity preferably about 100 cycles per second. The said moving coil is preferably damped e. g. with rubber.

Fig. 6 is a diagrammatical showing of a system such as has been described, illustrating the cooperative relation of the several elements.

Light from the light source L is projected in the form of a ribbon-like beam through the mask S onto the oscillograph mirror 0 whence it is reflected onto the film F. The mask is as shown in Fig. 5a and has three obscured portions of different heights or amplitude ranges. This mask S is moved up and down in dependence upon sound volume by the electro-magnetic movement B. With the mask in the position shown, so that the light beam position relative to the mask corresponds to Q of Fig. So, there are three cutting edges in use and therefore two tracks. If the sound amplitude changes to lower the mask, the conditions of R (Fig. 5a) will be obtained, namely five cutting edges and three tracks. If the sound amplitude changes to raise the mask, condition P (Fig. 511) will be obtained, giving one cutting edge and one track. The mask moves up and down as indicated by the double headed arrow above it. The oscillograph mirror O oscillates about its own axis as indicated by the double headed curved arrow, and in consequence the light image on the film (whether it comprises one, three, or five cutting edges) moves to and fro at sound frequencies in a direction at right angles to the direction of motion of the film, i. e. as indicated by the double headed arrow on the film. The film motion is represented by the arrow by the side of the film. The small broken line rectangles across the mask and across the film represent the recording light beam or ribbon.

With film recorded by this apparatus there will always be an odd number of cutting edge tracks (one, three, or five) on the film; but, as regards picking up of the sound from the film it is as though there were only one sound wave track, since the edges of the additional track or tracks (one for condition Q and two for condition R) cancel out. At the same time good silent recording is obtained since the additional track or tracks insure that ground noise due to film grain etc., is kept low for low amplitudes-,ln other words, under all conditions of recording, there is very little unexposed film. If the additional sound tracks were notprovided the film would be "noisy for low sound amplitudessince there would be a great deal of white or unexposed film to give rise to grain or like noise.

The sound frequencies to be recorded are indicated as derived through a microphone T and fed to an amplifier A, the output ofwhich is directly connected to the oscillograph C for actuating the mirror in accordance with the sound frequencies. The output of amplifier A is also connected to a detector or rectifier D, from which current varying in accordance with the amplitude of the sound is applied to the electro-magnetic movement B, high frequencies of the rectified signal current being filtered out. By such an arrangement, the mask S Will follow amplitude variations of the sound very accurately provided components of not more than 20 or 30 cycles per second are employed. This does not mean, however, that the different tracks will be cut in and out as often, for the mask is capable of limited movement within separate ranges of volume variation during which the added exposure is constant, and accordingly the additional tracks will be cut in and out at only a fraction of this frequency, practically and in fact at a sub-audible frequency. This, in addition to the shading and fading features already described, makes for substantially noiseless reproduction.

It is not thought necessary in this specification to illustrate in detail the recording oscillograph nor any of the optical devices for projecting and focussing the recording light on the film since this apparatus is all well known per se and is operated in the usual way. Again since the rectifier providing the unidirectional output for moving the mask, and the moving coil arrangement actually employed to effect such movement, and also the filter, do not per se form part of this invention and can be constructed by those skilled in the art without further description herein, it is not thought necessary to illustrate these in any more detail.

Practical dimensions in thousandths of an inch are indicated at the top of Fig. c. The invention is, of course, not limited to the use of these particular dimensions.

What we claim is:-

1. A variable area sound film having, over portions of its length corresponding to sound above a predetermined level, a single sound track bounded by a cutting edge track whose mean line is substantially centrally positioned with respect to the available record space on said film, and, over portions of its length corresponding to sound at or below said predetermined level at least one additional sound track to the side of and additional to said single sound track, said additional sound track being bounded by a pair of cutting edge tracks parallel to said first mentioned cutting edge track, said tracks being spaced.

2. A film in accordance with claim 1, further characterized in that over portions of the length corresponding to said predetermined level but above a second, lower predetermined level, there is provided said one additional sound track only, but for portions of the length corresponding to sounds at or below said lower level,- there is provided at least one further additional sound track similarly bounded by parallel cutting edge tracks, said first mentioned additional track being nearest said single track and said tracks being spaced.

3. A film in accordance with claim 1, wherein the additional track is faded in and out.

4. A film in accordance with claim 1, wherein the additional track is shaped in and out, being of constant width substantially throughout the length thereof.

5. The method of directly recording sound waves in a record strip on a light sensitive member which comprises exposing the light sensitive member along one side of the strip to a light beam varied in response to the form and amplitude of said waves, exposing portions of the unexposed area of the strip to a light beam of constant width, and controlling said beam of constant width in accordance with the amplitude of said waves.

6. The method of directly recording sound Waves in a record strip on a light sensitive member which comprises exposing the light sensitive member along one side of the strip to a light beam varied in response to the form and amplitude of said waves, and exposing constant width portions of the unexposed area of the strip to a light beam in steps in accordance with predetermined ranges of the amplitude of said waves.

7. The method of directly recording sound waves in a record strip on a light sensitive member as set forth in claim 5, wherein the light sensitive member is exposed simultaneously to the varied light beam and the light beam of constant width, with the light beam of constant width subject to displacement simultaneously with the variations of the other light beam in response to the form and amplitude of said waves, said light beam of constant width being controlled in accordance with the amplitude of the sound wave record relative to the width of the record strip.

8. The method of directly recording sound waves in a record strip on a light sensitive member as set forth in claim 6, wherein the light sensitive member is exposed simultaneously to both light beams with all the exposed portions of the record strip varied in response to the form and amplitude of said waves, the exposed portions of constant width being limited in accordance with the amplitude of the sound wave record relative to the width of the record strip.

9. Apparatus for recording sound waves comprising a light sensitive member having a record strip, means for exposing said record strip in variable widths in accordance with the form and amplitude of the sound waves, means for simultaneously exposing in constant width at least a portion of said record strip in spaced relation to the variable width exposed portion, and means for controlling the operation of the last said means in accordance with the amplitude of the sound wave record of variable width relative to the width of the record strip, said constant width portion being exposed in steps in accordance with the width of the unexposed portion of the record strip as determined by the relative amplitude of the sound wave record.

10. Apparatus for recording sound waves comprising a light sensitive member having a record strip, a light source, a recording oscillograph interposed between said light source and said record strip, means for forming light from said source into a ribbon beam, a mask device interposed in the path of said beam and having an odd plurality of cutting edges related to said beam, one of said cutting edges being adapted the simultaneous exposing means includes means for fading in and out the constant width portions exposed.

12. Apparatus as set forth in claim 9 wherein the simultaneous exposing means includes means for shaping in and out the constant width portions exposed, said portions being of constant width substantially throughout the length thereof.

HENRY JOSEPH ROUND. FRANK PERCIVAL SWAN'N. FRANK ARTHUR ROUND. 

